The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
As this is known per se, an aircraft propulsion unit conventionally comprises a turbojet engine housed within a nacelle.
The nacelle generally has a tubular structure comprising an air intake upstream of the turbojet engine, a middle section intended to surround a fan of the turbojet engine and its casing, a downstream section intended to surround the combustion chamber of the turbojet engine and casing, if appropriate, thrust reversal means. It may be terminated by an ejection nozzle the outlet of which is located downstream of the turbojet engine.
The assembly is fastened to a fixed structure of the aircraft, in particular under a wing or a fuselage, by means by means of a pylon or an attachment mast of the turbojet engine fastened to the latter in its front and rear portion by suspensions and which also provides the holding the nacelle.
There are numerous connection systems between the turbojet engine and the pylon so as to take the best the thrust forces of said turbojet engine. Documents FR 2 948 636, FR 2 948 635, EP 2 221 249, FR 2 892 706, FR 2 855 494, FR 2 755 942 are particularly cited.
Modern nacelles are intended to house a double flow turbojet engine able to generate, by means of the blades of the rotating fan, a hot air flow (also known as primary flow) coming from the combustion chamber of the turbojet engine, and a cold air flow (secondary flow) that flows outside the turbojet engine through an annular passage, also called stream, formed between a fairing of the turbojet engine (fixed inner structure or IFS being able to belong to the nacelle) and an inner wall of an outer structure of the downstream section of the nacelle (OFS or outer fixed structure). The two air flows are ejected from the turbojet engine to the back of the nacelle.
As mentioned previously, the outer fixed structure can house a thrust reversal device. The role of a thrust reverser during the landing of an aircraft is to improve the braking capacity thereof by redirecting forward at least one portion of the thrust generated by the turbojet engine. In this phase, the inverter blocks the stream of the cold flow and directs the latter toward the front of the nacelle, thereby generating a counter-thrust that is added to the braking of the wheels of the aircraft.
The means implemented to achieve this cold flow reorientation vary depending on the thrust-reverser type. However, in all cases, the structure of a thrust-reverser comprises movable cowls displaceable between, on the one hand, a deployed position in which they open a passage within the nacelle intended for the diverted flow, and on the other hand, a retracted position in which they close this passage and provide the inner and outer aerodynamic continuity of the nacelle.
These cowls can directly fulfill a function of deflection or simply actuation of other diverting means (inner doors).
In the case of a thrust reverser with grids, also known under the name of cascade-type thrust reverser, the reorientation of the air flow is carried out by diverting grids, the cowl having only one simple sliding function aiming at uncovering or covering these grids. Complementary blocker doors, also called blocking flaps, activated by the sliding of the cowling, allow at least one partial obstruction of the stream downstream of grids so as to optimize the reorientation of the cold flow.
In order to support the reversal movable cowls and to connect the downstream section to the rest of the nacelle, and in particular to the middle section by means of the fan casing, this one comprises fixed elements and in particular longitudinal beams connected upstream to a substantially annular assembly called front frame, formed in one or more portion(s) between said longitudinal beams, and intended to be fixed to the periphery of the downstream edge of the fan casing of the engine.
This front frame is connected to the fan casing by fixing means generally of the knife/groove type comprising a substantially annular flange, integral with the front frame and cooperating with a J or V-shaped groove, commonly called J-Ring.
The upper longitudinal beams are also connected to the pylon.
This structure can also be applied to a nacelle called smooth nacelle, wherein the downstream section constitutes an outer fairing of the nacelle and is not equipped with a thrust reversal device sliding along beams.
Thus, the downstream section is fastened, on the one hand, to the turbo jet engine by means of the fan casing, and, on the other hand, to the pylon.
In the case of an architecture of a conventional nacelle called nacelle with duct in C or in D (C-duct or D-duct), the downstream section has half-cowls with side opening (at the end of maintenance) by pivoting the upper beams around a substantially longitudinal axis of the nacelle extending along the fastening pylon of the turbojet engine.
There is also another type of nacelle architecture, more recent, called duct in O (O-duct) and particularly described in the document FR 2 916 426.
In this O-duct architecture, the downstream section does not comprise two half-cowls with side opening anymore, but a single-piece cowl substantially peripheral, and which extends from one side to the other of the pylon.
For maintenance purposes, such a cowl can't be opened by pivoting and is movably mounted by sliding towards the back of the nacelle, along the rails or slides disposed on either side of the pylon.
For classical nacelle architecture in C or in D, the nacelle/pylon connections have no particular difficulty and are well known. For the downstream section, the pivoting mounting of the cowls on the pylon, allows particularly good accommodation of the relative displacements and other mounting sets of the assembly.
This does not hold true for an O architecture in which these adjustment means no longer exist. An example of a connection system for an O-duct architecture is described in US 2011/0023450 document.
The rails and slide of the downstream structure on the pylon are also structural and must thus provide the holding of the assembly, the resumption of the forces and their transmission to the pylon.
Such an architecture and connection lead to many implementation difficulties.
More specifically, as mentioned above, a downstream section of the nacelle of the O-duct type is attached, on the one hand, to the turbojet engine by a front frame connected to the fan casing (interface called A2), and on the other hand, to the pylon by means of its slide rails.
These rails are oriented substantially at 90° with respect to the interface A2 of connection to the fan casing.
This double attachment generates a hyperstatic assembly and a major difficulty is to accommodate, between the turbojet engine and the pylon, the assembly tolerances, the related displacements under loads as well as related displacements due to the thermal expansion of the turbojet engine, among others.
Currently, these related displacements must be taken up by the flexibility of the fixed structure of the downstream section. This requires an adaptation of the materials used.
The existing solutions that address this problem of nacelle/pylon connection regard the conventional nacelles with structures in C or in D, and are not adapted to a nacelle with an O structure having a direct connection to the pylon (sliding O nacelle). Thus, there is a need for a connection which allows solving this problem.